Download Stereoisomeric Behaviour of Octahedral [[6]MØ4] Chains

Stereoisomeric Behaviour of Octahedral [[6]MØ4] Chains
Aaron J. Lussier* and Frank C. Hawthorne
Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2
[email protected]
Infinite chains of edge-sharing octahedra with the general stoichiometry [[6]MØ4] (where Ø = O2-,
(OH)-, (H2O), Cl-, or F-) are present in over 175 mineral species with over 100 crystal structure
types. These chains may be decorated with varying numbers of TØ4 tetrahedra and DØ3
triangles such that the complete stoichiometry of the corresponding mineral may be expressed
as AvMw(TØ4)x(DØ3)yØz where A = a low-charge highly-coordinated cation, commonly an alkali
or alkali earth, that is only weakly bonded to the chain, M = a high-charge cation in octahedral
coordination, T = a high-charge cation in tetrahedral coordination, and D = a cation in triangular
coordination. From a combinatorial perspective, a very large number of chain stereoisomers is
possible; however, only a relatively small number of these chains are observed in actual
minerals. Rather than forming a large number of structures with a large number of
stereoisomers, we can see that Nature uses a small number of stereoisomers to construct a
large number of structures by varying the degree and dimensionality of connectivity between the
units, with rarely more than one chain type observed in a single structure.
Within a [[6]MØ4] chain, the shared edges of an octahedron may by related by the inversion
centre, forming straight trans-trans chains, or by other symmetry operators, forming more
complex, kinked chains with a mixture of successive trans-cis linkages. On average, the transtrans chains have repeat lengths of two octahedra, corresponding to distances of ~6 Å, whereas
the mixed trans-cis chains have variable repeat lengths. Either type of chain may occur isolated
in the structure, bonded only to A-group cations. However, the relatively simple geometry of the
trans-trans chains allows certain stereoisomers to link together directly, forming progressively
more dense structures. This is exemplified by the behaviour of the trans-trans M(TØ4)Ø2 chain,
which occurs in over 50 different mineral species, all with chain axes oriented parallel to each
other. This chain structure consists of (TØ4) groups linking two apical oxygens of every second
pair of adjacent octahedra. The [M+M]Ø anions are typically underbonded and may easily bridge
to additional tetrahedra. Where the additional tetrahedron is not part of another unit, the chains
are isolated, as is observed in the structures of brackebuschite and bushmakinite, amongst
others. However, where (TØ4) groups link to the [M+M]Ø of a second chain and, in turn, the (TØ4)
groups of the second chain link to the [M+M]Ø of the first chain, infinite sheets result by translating
this configuration along one direction, and each chain is linked to two others. This is observed in
the structures of tsumcorite and bermanite, amongst others. Conversely, where the (TØ4)
groups of the second chain bond to the [M+M]Ø of a third chain, usually with inter-chain bond
directions differing by ~90°, three-dimensional frameworks result by translating this
configuration along the two directions, and each chain is linked to four others. This is observed
in the structures of adelite and descloizite, amongst others. Hence, by considering the
occurrence of topologically similar structural units, direct connections can be made between
large numbers of minerals of complex structure.
GeoCanada 2010 – Working with the Earth
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